Kappa are reptilian humanoids that live in bodies of fresh water throughout Japan and are particularly common in the countryside. Their appearance varies by region, but their most common features are a beak, a shell, and a sara (plate), a flat, hairless region on top of the kappa's head that is the source of its power. The kappa must keep water, preferably from the body of water they call home, within this part of their head or else it risks weakening and possibly even dying.

Kappas are usually portrayed as tricksters whose behavior ranges from mischief such as looking up women's kimonos to more harmful acts such as drowning children and animals, kidnapping children, raping women, and eating human flesh. They may down people to drink their blood, eat their livers, or gain power by taking the person's shirikodama (a mythical ball said to contain the soul that is located in the anus). Children born because a kappa raped a woman are said to be repulsive and are usually buried immediately after birth.

If someone catches a kappa trying to drown an animal, they can be made to apologize, sometimes in writing since they can learn human languages. In fact, kappa are quite intelligent. They are said to have taught humans about medicine and how to set bones. They enjoy sports such as sumo wrestling and games of skill such as shogi (also known as Japanese chess).

Kappa are so well known for drowning people that there are still signs in Japan warning people about them near bodies of water. This helps encourage children to be careful when they play in the water.

There are a few things you can do to discourage a kappa from harming you or to defend yourself from one. For example, kappa absolutely love cucumbers. If you write your name onto a cucumber and toss it into the water where the kappa lives, it will allow you to bathe. Since kappa love the taste of human flesh, giving a kappa a cucumber will ensure your temporary safety. Kappas also have an aversion to iron, sesame, and ginger, so they can easily be driven away using these substances.

You can also challenge the kappa. Kappa are honorable creatures that keep all promises they make, so making one promise to leave you alone if it can't complete a challenge you set for it will make it leave you in peace. Another way to guarantee your safety is to trick the kappa into spilling all of the water from its sara. Kappa love to wrestle, so you can easily make the kappa spill its water as you try to beat it in a wrestling match. Kappa are polite and will bow back if you bow to them, so they can easily be tricked into bowing deeply enough to spill all of their water. If you refill the kappa's sara, it will be a loyal friend to you for the rest of your life. It is also possible to befriend a kappa by giving it nasu (Japanese eggplant), soba (buckwheat noodles), nattō (fermented soybeans), or kabocha (winter squash), but these are not as likely to work as cucumbers.

Once befriended, a kappa is willing to perform a variety of tasks for people such as irrigating their land and bringing them fresh fish. They are also willing to befriend lonely children and play sports and games with adults.

Some shrines have been built to honor especially helpful kappa where people can leave them cucumbers as offerings. Festivals are held to placate kappa in the hopes of a good harvest. These festivals usually take place on the equinoxes when kappa are said to travel from the rivers to the mountains and vice versa.

FUN FACT: Kappa are so well known for loving cucumbers that cucumber rolls are called kappamaki.

Examples in various media:

Kapp'n (Animal Crossing, Japanese versions only)

Although he is correctly called a kappa in Japanese versions of the Animal Crossing games, he is called a turtle in all other versions. This change makes some sense since Western players wouldn't necessarily know what a kappa is, but Kapp'n is very clearly a kappa, not a turtle. Here is a picture that shows his sara:

As you can see, Kapp'n has the trademark's trademark area on his head. It's made even more obvious that he is a kappa by the fact that he sings about loving cucumbers. Unfortunately, the lack of widespread knowledge about Japanese folklore, especially among children, ensures that Kapp'n is mislabelled outside of Japan.

Kaatan (Kappa no Kaikata, Translation: How to Raise Kappa)

Coo (Summer Days with Coo)

Walkappa (Yo-kai Watch, known as Nogappa in Japanese)